X-ray Scintillation in Lead Halide Perovskite Crystals

M. D. Birowosuto,1,2,* D. Cortecchia,3,4 W. Drozdowski,5 K. Brylew,5 W. Lachmanski,5

A. Bruno,4 C. Soci2,4,6,*

1CINTRA UMI CNRS/NTU/THALES 3288, Research Techno Plaza, 50 Nanyang Drive,

Border X Block, Level 6, Singapore 637553

2Center of Disruptive Photonic Technologies, TPI, SPMS, Nanyang Technological

University, 21 Nanyang Link, Singapore 637371

3Interdisciplinary Graduate School, Nanyang Technological University, Singapore 639798

4 Energy Research Institute @ NTU (ERI@N), Research Techno Plaza, Nanyang

Technological University, 50 Nanyang Drive, Singapore 637553

5Institute of Physics, Faculty of Physics, Astronomy, and Informatics, Nicolaus Copernicus

University, Grudziadzka 5, 87-100 Torun, Poland

6 School of Physical and Mathematical Sciences, Division of Physics and Applied Physics,

Nanyang Technological University, 21 Nanyang Link, Singapore 637371

*Corresponding authors: mbirowosuto@ntu.edu.sg; csoci@ntu.edu.sg

Keywords: X-ray scintillators, organometallic perovskites, single crystals, thermoluminescence

mailto:mbirowosuto@ntu.edu.sgmailto:csoci@ntu.edu.sg

1

Abstract

Current technologies for X-ray detection rely on scintillation from expensive inorganic crystals

grown at high-temperature, which so far has hindered the development of large-area scintillator

arrays. Thanks to the presence of heavy atoms, solution-grown hybrid lead halide perovskite

single crystals exhibit short X-ray absorption length and excellent detection efficiency. Here we

compare X-ray scintillator characteristics of three-dimensional (3D) MAPbI3 and MAPbBr3 and

two-dimensional (2D) (EDBE)PbCl4 hybrid perovskite crystals. X-ray excited

thermoluminescence measurements indicate the absence of deep traps and a very small density

of shallow trap states, which lessens after-glow effects. All perovskite single crystals exhibit

high X-ray excited luminescence yields of >120,000 photons/MeV at low temperature. Although

thermal quenching is significant at room temperature, the large exciton binding energy of 2D

(EDBE)PbCl4 significantly reduces thermal effects compared to 3D perovskites, and moderate

light yield of 9,000 photons/MeV can be achieved even at room temperature. This highlights the

potential of 2D metal halide perovskites for large-area and low-cost scintillator devices for

medical, security and scientific applications.

2

Introduction

The investigation of X-ray detectors started with the discovery of X-rays by Wilhelm Röntgen,

who noticed the glow from a barium platino-cyanide screen placed besides a vacuum tube [1,2].

Since this discovery, more than one hundred years ago, the development of efficient [3-5] and

large-area [5-7] X-ray detectors has been a topic of continuous interest, targeting a wide range of

applications, from crystallography [8] to space exploration [9].

Modern X-ray detectors rely on two main mechanisms of energy conversion. The first is

photon-to-current conversion, in which a semiconducting material directly converts the incoming

radiation into electrical current [4-6]; the second is X-ray to UV-visible photon down-

conversion, in which a scintillator material is coupled to a sensitive photodetector operating at

lower photon energies [2]. Both methods are equally compelling for practical implementations,

although their viability will ultimately depend on the development of new materials to overcome

some of the current limitations, such as high cost, small area, and low conversion efficiency of

the X-ray absorbers. Recent demonstrations of the use of hybrid metal-halide perovskites for X-

and -ray detection has spurred great interest in this class of materials [7,10,11,12]. Besides their

good detection efficiency, solution processing holds great promise for facile integration and

development of industrial and biomedical applications.

Methylammonium lead trihalide perovskites (MAPbX3 where MA=CH3NH3 and X= I, Br, or

Cl) have demonstrated excellent performance in optoelectronic devices like field effect

transistors [13], highly sensitive photodetectors for visible region [14], and light emitting devices

[15,16]. Moreover, compositional tuning was used to realize tunable-wavelength lasers [17]. As

X-ray detectors, MAPbX3 yield notably large X-ray absorption cross section due to large atomic

numbers of the heavy Pb and I, Br, Cl atoms [10,11]. Thin-film MAPbX3 p-i-n photodiode and

3

lateral photoconductor devices have shown good efficiency for X-ray photon-to-current

conversion [10,11]. However, thin-film X-ray detectors have typically low responsivity at high

(keV) photon energies, where the absorption length (~mm) is much larger than the film thickness

(~μm); even if thickness is increased to improve detection probability, direct photon-to-current

conversion is ultimately hampered by the limited carrier-diffusion length (~1 μm in perovskites)

[10]. Efficient X-ray photon-to-current conversion has been shown recently in single-crystal

(thick) perovskite MAPbBr3, but sensitivity is still limited to energies up to 50 keV [11]. Also,

standard -photon counting for energies up to 662 keV has been demonstrated in MAPbI3 [12].

As opposed to direct photon-to-current conversion detectors, X-ray scintillators do not suffer

from limited carrier diffusion length of the absorbing material [18,19]. Thin films of

phenethylammonium lead bromide, PhE-PbBr4, with sub-nanosecond scintillation decay time

have been previously tested in X-ray [20] and proton [21] scintillators, but yielded only 5-6%

detection efficiency of 60 keV X-rays, limited by the film thickness (200 μm) [21]. By

combining the good high-energy response with large absorption cross section deriving from

large thickness and high mass-density, single crystal perovskite scintillators are therefore

expected to improve detection efficiency of keV X- or γ-rays.

In this paper, we present a thorough comparative study of the scintillation properties of three-

dimensional (3D) and two-dimensional (2D) low-bandgap perovskite single crystals. We have

synthesized mm-scale 3D perovskite crystals MAPbI3 and MAPbBr3, and 2D perovskite crystal

(EDBE)PbCl4 (EDBE=2,2’-(ethylenedioxy)bis(ethylammonium)), comprising of alternating

organic and inorganic layers which form a multi-quantum-well-like structure. The excellent

quality of these crystals is indicated by structural analysis and by the very small density of

shallow traps (n0~105-107 cm-3, E~10-90 meV) determined by X-ray excited

4

thermoluminescence, which reduces after-glow effects. Thanks to their lower bandgap compared

to traditional scintillator crystals [6], perovskite crystals produce extremely high light yields of

>120,000 photons/MeV (as estimated from X-ray-excited luminescence) at low temperature. In

3D perovskites, the light yield is greatly reduced at room temperature (

5

MAPbBr3, and (EDBE)PbCl4 crystals appear lustrous black, orange, and white, respectively. The

corresponding glows under ultraviolet lamp excitation are green and white for MAPbBr3 and

(EDBE)PbCl4 crystals, while the glow of MAPbI3 could not be observed since its emission lies

in the near infrared. Crystal colors and glows agree well with the absorption and

photoluminescence properties of the corresponding thin films, which show optical energy gaps

of Eg=1.51, 2.18, and 3.45 eV for MAPbI3 [22], MAPbBr3 and (EDBE)PbCl4 [24], respectively

(Supplementary Fig. S4).

Perovskite crystals offer multiple advantages for X-ray scintillation, specifically: i. Since the

light yield of X-ray scintillation is inversely proportional to the optical bandgap Eg [2,18], low-

bandgap perovskites of MAPbI3, MAPbBr3 and (EDBE)PbCl4 are expected to yield up to about

270,000, 190,000, and 120,000 photons/MeV, respectively. Those light yields are much higher

than state-of-art cerium (Ce3+) doped lanthanum tribromides LaBr3 (Eg=5.90 eV) [25,26] and

Ce3 + doped lutetium iodides LuI3 (Eg=4.15 eV) [27,28] scintillators, with light yields of 68,000

and 100,000 photons/MeV, respectively. ii. Since X-ray absorption length scales with the

effective atomic number Zeff and mass density ρ [2], MAPbI3, MAPbBr3, and (EDBE)PbCl4

(Zeff=66.83, 67.13, and 67.52, ρ=3.947, 3.582, and 2.191 gr⁄cm3, respectively) should reach X-

ray absorption lengths up to 1 cm at 1 MeV, similar to Ce3+-doped LaBr3 and LuI3 scintillators

(see Supplementary Fig. S5). iii. The unusually large Stokes shift of two-dimensional

(EDBE)PbCl4 could be particularly beneficial to the scintillation yield [29], which is

substantially reduced by self-absorption of the luminescence [30]. iv. The extremely fast

photoluminescence decay of MAPbI3, MAPbBr3, and (EDBE)PbCl4 (fast components of 4.3,

0.8-5.2, and 7.9 ns, respectively) may provide faster scintillation than 15 ns of Ce3+-doped LaBr3

[25,30] and 33 ns of Ce3+-doped LuI3 [28] (Supplementary Fig. S6). Nanosecond scintillation

6

decay times were indeed demonstrated in PhE-PbBr4 using X-ray and -ray pulses, consistent

with time-resolved photoluminescence [20,31]. v. Finally, long emission wavelengths in the

range of 400 to 700 nm allow optimal detection of scintillation using highly sensitive avalanche

photodiodes (APDs), which can reach quantum efficiencies up to 90-100% in comparison with

photomultipliers (PMTs) with only 40-50% efficiency [28].

Figure 1. Crystal structure and appearance. Top row: crystal structure

representation of MAPbX3 (X=I, Br) three-dimensional perovskites (left), and

(EDBE)PbCl4 two-dimensional perovskite (right); Middle row: photographs of the large

single crystals of hybrid lead halide perovskites; Bottom row: glow of the crystals under

ultraviolet lamp excitation. Scale bars: 5 mm.

7

Figure 2. Emission spectra under X-ray and optical excitation. X-ray excited

luminescence (light color area) and photoluminescence (dark color area) spectra of a)

MAPbI3, b) MAPbBr3, and c) (EDBE)PbCl4 recorded at room temperature with

excitation wavelengths for photoluminescence of 425, 500, and 330 nm, respectively.

Photoluminescence and X-ray excited luminescence spectra were normalized to their

maxima, and normalized X-ray excited luminescence spectra were divided by a factor of

two for clarity.

The X-ray excited luminescence and photoluminescence spectra of MAPbI3, MAPbBr3, and

(EDBE)PbCl4 crystals recorded at room temperature are shown in Fig. 2 (see experimental

details in Materials and methods section). Both X-ray excited luminescence and

photoluminescence spectra of MAPbI3 have a single broadband peak centered at 750 nm with

8

FWHM of ~80 nm (Fig. 2a). For MAPbBr3, both X-ray excited luminescence and

photoluminescence spectra exhibit double peaks centered around 560 and 550 nm, respectively.

MAPbBr3 has the narrowest emission band with full width of half-maximum (FWHM) of ~40

nm (Fig. 2b). On the other hand, (EDBE)PbCl4 has the broadest emission band centered at 520

nm, with FWHM of ~160 nm (Fig. 2c). Based on emission wavelength, MAPbBr3 and

(EDBE)PbCl4 appear to be the most promising candidates for the scintillators coupled to APD

[28].

In all perovskite crystals, X-ray excited luminescence and photoluminescence spectra are

very similar, indicating that the dominant scintillation mechanism is straightforward: upon X-ray

absorption, high-energy excitations thermalize through ionizations and excitations of atoms,

until excitons are generated at energies near the bandgap. X-ray excited luminescence stems

solely from the intrinsic excitonic emission of the perovskites, and no other defect states seem to

be involved in the scintillation process.

The dynamics of radiative processes in materials under high-energy excitation is often

complicated by slower non-exponential components due to charge carrier trapping and re-

trapping, which manifest themselves as delayed luminescence, or afterglow. Upon termination of

the X-ray excitation, afterglow effects would typically contribute a residual luminescence

background with characteristic lifetime of few ms, thus lowering the effective light yield and

worsening the signal-to-noise ratio. Afterglow effects are particularly detrimental for

applications like computed tomography, in which temporal crosstalk considerably reduces the

image quality [2]. Charge carrier trapping and re-trapping processes can be monitored by

thermoluminescence measurements. In our specific mode of operation for thermoluminescence

intensity measurements, we were able to record steady-state X-ray excited luminescence

9

intensity during irradiation, immediately prior to the thermoluminescence scan (see details in

Materials and methods). In this way, two distinct integrated intensities can be evaluated: the first

one, which we denote as ITL, comprising the range from the end of X-ray irradiation till the end

of the entire run, while the second one, denoted as ITL+IssXL, comprising the range from the start

of the X-ray irradiation until the end of the run. This allows calculating, for each sample, the

ITL/(ITL+IssXL) ratio, which can be interpreted as the fraction of the total excitation energy

accumulated into traps [19,32]. The value of this ratio, therefore, provides a qualitative estimate

of the influence of traps on the scintillation yield.

Figure 3. Residual luminescence background after X-ray excitation. Low

temperature thermoluminescence curves of a) MAPbI3, b) MAPbBr3, and c)

(EDBE)PbCl4. The data are presented on a time scale starting at temperature of 10 K and

increasing to 350 K after 3600 s, as indicated by the dashed line in the right panel

(temperature scale on the right axes).

Typical thermoluminescence curves of the metal halide perovskite crystals are shown as solid

curves in Fig. 3. After termination of the X-ray excitation at 10 K, long tails extending to

thousands of seconds were observed in all crystals. Although the long-lived component of this

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afterglow effect is much slower than the photoluminescence decay (see Supplementary Fig. S6),

it only occurs at low temperatures (~10 K) and is negligible at room temperature. In the case of

MAPbI3 and MAPbBr3, low temperature thermoluminescence curves are dominated by a double-

structured peak, with two smaller satellite peaks appearing at longer times (Figs. 3a and 3b). In

(EDBE)PbCl4, the low-temperature thermoluminescence curve shows that one peak strongly

dominates the other peak while the total intensity of the peaks is much higher than those in

MAPbI3 and MAPbBr3 (Fig. 3c). The ratio of ITL/(ITL+IssXL) ~ 0.002 is very similar in both

MAPbI3 and MAPbBr3, which is extremely low in comparison with other oxide materials used

for scintillators, such as lanthanide aluminium perovskite or garnets [19,32,33,35]. Moreover,

MAPbI3 and MAPbBr3 crystals show nearly trap-free behavior from T=75 K up to the highest

temperature investigated of T=350 K, a very desirable characteristic from the point of view of

scintillation speed and efficiency. In (EDBE)PbCl4, ITL/(ITL+IssXL) ~ 0.058, a slightly higher

value than in the three-dimensional perovskite crystals, but still relatively low.

Figure 4. Determination of low-energy trap states. Glow curves of a) MAPbI3, b)

MAPbBr3, and c) (EDBE)PbCl4 recorded after 10 min X-ray irradiation at 10 K, at a

heating rate of 0.14 K/s. The solid lines are the best fits to the experimental data points by

multiple Randall-Wilkins equations (Eq. 1): single components and peak temperatures

(Tmax) are indicated by dashed lines and arrows, respectively (see Table 1 for fitting

parameters).

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The zero-order glow curves of the three crystals are presented in Fig. 4. Appearance of

thermoluminescence signal at temperatures below 150 K reveals the existence of low-energy trap

states. Since for such states it is difficult to determine the exact number of traps, their depth and

frequency factors [33], we restrict our analysis to thermoluminescence peaks with intensity

larger than 10 % of the maximum. Thermoluminescence curves have been deconvoluted into k

glow peaks, based on the classic Randall-Wilkins equation [34]:

𝐼𝑇𝐿 = ∑ 𝑛0𝑖𝑉𝜎𝑖exp (−𝐸𝑖

𝑘𝐵𝑇)

𝑘

𝑖=1

exp (−𝜎𝑖𝛽

∫ exp (−𝐸𝑖

𝑘𝐵𝑇′) 𝑑𝑇′

𝑇

𝑇0

) (1)

where T is the temperature, β the heating rate, and kB the Boltzmann constant; n0i is the initial

trap concentration, V is the crystal volume, Ei the trap depth, σi the frequency factor of each

component. Note that the unitless number of traps n0iV is often used to compare the afterglow of

different crystals [19,32,33,34,35].

This analysis provides a good indication of the characteristics of prevailing trap states,

however it cannot resolve the existence of traps that fall at times much longer than seconds, or

with mixed order kinetics [32]. The room temperature lifetime of trapped carriers, such as

electron or hole centers and excitons, τi, can also be estimated from the energy and frequency

factor of the trap, using the well-known Arrhenius formula:

1

𝜏𝑖= 𝜎𝑖exp (−

𝐸𝑖𝑘𝐵𝑇

) (2)

While the glow curves of MAPbI3 and MAPbBr3 in Fig. 4a and b have been fitted using four

and three components, respectively, the glow curve of (EDBE)PbCL4 in Fig. 4c could be fitted

with only two components. The corresponding fitting parameters are shown in Table 1. All

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crystals have relatively low trap densities, with depth energy (E) varying from ~10 to 90 meV.

The initial trap concentrations n0 in MAPbI3 and MAPbBr3 can be calculated from the total

number of traps (n0V~103-104) and the volume of the crystal (V~30-100 mm3). The resulting trap

concentrations (n0~105-107 cm-3) are comparable to those of shallow traps previously observed in

photoconductivity measurements (~105-107 cm-3) [11] and space-charge-limited-current (~109-

1010 cm-3) [23], also considering the uncertainty in the estimate of the active crystal volume. The

fastest room temperature lifetimes (τ) of MAPbI3 and MAPbBr3 are of the order of milliseconds,

long enough to contribute to the light yield components without residual luminescence

background. Correspondingly, logarithmic frequency factors (ln σ) are all below 16, which is

much smaller than ln σ~30 typically found in pristine or activated oxide materials [19,32,33,35],

also reported in Table 1 for comparison. (EDBE)PbCl4 has the largest trap density among the

perovskites we investigated, n0~107 cm-3. Large concentration of shallow traps may be beneficial

for X-ray scintillation at low-temperature, as in the case of Ce3+-doped YAlO3 and LuAlO3 [35],

or pristine Li2B4O7 [36]. This is indeed seen in temperature dependent X-ray excited

luminescence spectral maps shown in Fig. 5.

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Compound

Tmax (K) E (eV) ln σ (s-1) τ (s) n0V Reference

MAPbI3

32 0.0309 8.09 1.04⋅10-3 2.45⋅104

This work 46 0.0226 1.78 0.41 1.85⋅104

56 0.0901 15.60 5.95⋅10-4 6.12⋅103

62 0.0389 3.25 0.18 1.45⋅104

MAPbBr3

33 0.0139 1.16 0.54 7.61⋅104 This work 56 0.0602 9.02 1.31⋅10-3 2.10⋅104

68 0.0909 12.1 2.04⋅10-4 2.73⋅104

EDBEPbCl4 32 0.0177 2.83 0.12 5.95⋅105 This work

45 0.0281 3.40 0.10 1.71⋅106

LuAlO3: Ce3+

36 0.0148 0.9346 2.16⋅10-2 2.84⋅104

[19,32,35]

88 0.105 10.07 2.29⋅10-2 1.53⋅104

187 0.498 27.22 2.51⋅10-2 2.10⋅106

206 0.385 17.56 1.61⋅10-2 4.64⋅104

223 0.669 30.99 2.18⋅10-2 1.38⋅104

253 0.75 30.53 2.08⋅10-2 4.84⋅104

273 0.799 30.08 2.05⋅10-2 1.52⋅105

YAlO3: Ce3+

108 0.30 29.24 4.99⋅10-2 ~105 [19,32,35] 154 0.50 34.18 3.02⋅10-2 ~105

281 0.421 18.05 1.95 ~104

Gd3Al2Ga3O12: Ce3+

36 0.0576 15.9 1.2⋅10-6 1.6⋅105

[33]

45 0.0446 8.32 1.4⋅10-3 4.7⋅105

73 0.116 15.1 2.7⋅10-5 3.4⋅105

181 0.211 9.01 0.52 2.9⋅105

240 0.527 21.31 0.65 2⋅105

255 0.321 9.76 19 7.5⋅105

Table 1. Trap state parameters. The parameters were derived from the fitting of first-

order glow curves in Figure 4: Tmax is the temperature at which the glow curve peaks, E

the trap depth, ln σ the logarithmic frequency factor in s-1, τ the room temperature

lifetime, and n0V the total, initial number of traps. Comparative parameters of known

scintillator materials from the literature are also reported in the last three lines.

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Figure 5. Temperature dependent luminescence under X-ray excitation. X-ray

excited luminescence spectra (X-ray excited luminescence) of perovskite crystals at

various temperatures, from 10 to 350 K: (a) MAPbI3, (b) MAPbBr3, and (c)

(EDBE)PbCl4. (d) Comparison of the normalized X-ray excited luminescence spectra at

T=10 K.

MAPbI3 (Fig. 5a) and MAPbBr3 (Fig. 5b) show strong dependence of X-ray excited

luminescence on temperature, with significantly reduced emission at temperatures above 100 K.

At very low temperatures they display distinct emission bands with sharp maxima at 770 nm and

540 nm, respectively (see Fig. 5d for comparison of X-ray excited luminescence spectra recorded

15

at T=10 K). Notably, the X-ray excited luminescence peak at 770 nm, with FWHM of 5 nm, has

the same characteristics of coherent light emission previously observed in MAPbI3 [17]. Side

bands also appear at 760 and 800 nm in MAPbI3 and at ~590 nm in MAPbBr3, but their origin is

still unclear. The X-ray excited luminescence spectrum of (EDBE)PbCl4 (Fig. 5c) consists of a

much broader band peaking at ~520 nm, with intensity significantly less sensitive to temperature.

As temperature increases, the X-ray excited luminescence intensity first decreases between 10

and 50 K, then increases towards 130 K, and reduces steadily at higher temperatures. In all

crystals, the FWHM of X-ray excited luminescence peaks increases with increasing temperature,

consistent with the spreading of excited electrons to high vibrational levels [37].

As discussed previously, the light yield of perovskite single crystals estimated from their

bandgaps should be >120,000 photons/MeV. From the pulse height spectra of samples excited

with 662 keV γ-ray of Cs137 shown in Supplementary Fig. S7, the actual light yield of

(EDBE)PbCl4 at room temperature is moderate, with ~9,000 photons/MeV. We note that they

are not so many reports about the energy spectra from γ-ray reported for perovskite scintillator

[10] and direct conversion detector [12]. Light yield of (EDBE)PbCl4 is actually similar to that

of two-dimensional perovskite crystal PhE-PbBr4 (10,000 photons/MeV) reported previously

[31]. The light yields of MAPbBr3 and MAPbI3 at room temperature are much lower, and cannot

be extracted from pulse height experiments. Low light yields at room temperature may arise

from thermally activated quenching effects. To confirm this hypothesis, we have derived light

yields at different temperatures from the integrated intensities of the X-ray excited emission

spectra in Fig. 5; considering the small afterglow, we expect the light yield derived from X-ray

excited emission spectra to be very similar to that derived from pulse height spectra.

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Figure 6. Temperature dependence of the light yields. Light yields of MAPbI3,

MAPbBr3, and (EDBE)PbCl4 obtained from the integrated X-ray excited luminescence

intensities at various temperatures, from 10-350 K. The left axis shows integrated

intensity in arbitrary units obtained from the corrected X-ray excited luminescence

spectra in Fig. 5, while the right axis exhibits the light yield in absolute units after

calibration of the light yield of (EDBE)PbCl4 at 300 K to ~9,000 photons/MeV, as

derived independently from its pulse height spectrum. The inset shows details of the

curves from 290 to 310 K.

Light yields derived from the integrated X-ray excited luminescence emission intensities of

the three halide perovskite crystals as a function of temperature are reported in Fig. 6. We

integrated the corrected intensity of X-ray excited luminescence spectra in Fig. 5 and used the

light yield of ~9,000 photons/MeV derived from the pulse height spectrum of (EDBE)PbCl4 at

300 K (Supplementary Fig. S7) to calibrate the integrated intensity. For (EDBE)PbCl4, the

resulting light yield at 300 K is about ~8% of the maximum at 130 K. Since the light yield is

linearly proportional to the photoluminescence quantum efficiency [18] while the efficiency of

charge transport to the recombination center is almost unity [24,29], the ratio of 8% is consistent

17

with reported (EDBE)PbCl4 photoluminescence quantum efficiency of less than 10% at room

temperature. Light yields of MAPbI3 and MAPbBr3 are

18

materials [19,33]. Low-temperature X-ray excited luminescence measurements have shown that

the X-ray luminescence yield can be as high as ~120,000 photons/MeV in (EDBE)PbCl4 at

T=130 K, and in excess of 150,000 photons/MeV in MAPbI3 and MAPbBr3 at T=10 K. The wide

synthetic versatility of hybrid perovskites allows easy tuning of their scintillation properties: for

example, their emission spectra can be controlled by cation or halide substitution to perfectly

match the spectral sensitivity of high-quantum-efficiency APD, like in the case of MAPbBr3 and

(EDBE)PbCl4. Their emissive properties can be further enhanced through engineering of

perovskite structure and dimensionality: while light yield of 3D perovskites MAPbI3 and

MAPbBr3 is significantly reduced at room temperature (

19

and 350 K. We note that the measurements were carried out from 350 K to 10 K to avoid

possible contributions from thermal release of charge carriers to the emission yield. After X-ray

excited luminescence measurements, we measured low temperature thermoluminescence and

glow curves. Prior to thermoluminescence measurements, the samples were exposed to X-rays

for 10 min at 10 K. Thermoluminescence and glow curves were recorded between 10 and 350 K

at a heating rate of about 0.14 K/s. Thermoluminescence curves were recorded with the

monochromator set to the zeroth order. Photoluminescence spectra were recorded with a

commercial spectrofluorometer HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer at room

temperature.

Crystal growth: Three-dimensional perovskite precursors, MABr and MAI, were synthetized by

mixing hydrobromic acid (48% wt in water, Sigma-Aldrich) and hydroiodic acid (57% wt in

water, Sigma-Aldrich) with methylamine solution (CH3NH2 , 40% in methanol, Tokyo Chemical

Industry, Co., Ltd) in 1:1 molar ratio. The ice-cooled mixture was left under magnetic stirring for

2 h, and the solvent removed with a rotary evaporator. The resulting powders were dissolved in

ethanol, crystallized with diethylether for purification repeating the cycle 6 times, and finally

dried in vacuum oven at 6 ○C for 12 h. For (EDBE)PbCl4(EDBE=2,2’-

(ethylenedioxy)bis(ethylammonium)), the organic precursor (EDBE)Cl2 was synthetized in

aqueous solution by reaction of 2,2’-(ethylenedioxy)bis(ethylamine) (98%, Sigma Aldrich) with

excess of HCl (37% in H2O). The solution was stirred for 4h at room temperature to complete

the reaction. A purification process similar to that discussed for MABr and MAI was applied to

collect the final white and high purity powders.

For the synthesis of hybrid perovskite crystals, the following inorganic precursors were

purchased from Sigma-Aldrich: lead(II) chloride (PbCl2, 99.999%), lead(II) bromide (PbBr2,

20

99.999%) and lead(II) iodide (PbI2, 99.0%). Crystals of MAPbBr3 were synthetized using inverse

temperature crystallization as similarly reported elsewhere [27]. 2 ml of 1M DMF solution of

MABr and PbBr2 (1:1 molar ratio) were left overnight on a hotplate (110 ○C) without stirring,

allowing the precipitation of the perovskite crystals. MAPbI3 were obtained by slow evaporation

at room temperature of a saturate N, N-dimethylformamide (DMF) solution of MAI and PbI2

(1:1 molar ratio). To obtain (EDBE)PbCl4 crystals, a 1M solution of (EDBE)Cl2 and PbCl2 (1:1

molar ratio) in dimethylsulphoxide (DMSO) was prepared by dissolving the precursors at 110 ○C

on a hotplate. After natural cooling of the solution at room temperature, slow crystallization over

a period of 1 month results in the formation of cm-scale white perovskite crystals. The

crystallization processes were performed under inert N2 atmosphere. All the obtained crystals

were collected from the precursor solutions, washed with diethylether and dried in vacuum

overnight.

Acknowledgements

Research was supported by the Ministry of Education (MOE2013-T2-044 and MOE2011-T3-1-

005) and by the National Research Foundation (NRF-CRP14-2014-03) of Singapore. X-ray

excited luminescence and thermoluminescence measurements were performed at the National

Laboratory for Quantum Technologies (NLTK), Nicolaus Copernicus University, supported by

the European Regional Development Fund.

Author contributions

MDB and CS conceived the idea. DC synthesized the perovskite precursors, prepared, and

characterized crystals and films. MDB and WD designed the experiments. KB and WL

performed X-ray excited luminescence and thermoluminescence measurements. DC and AB

21

collected absorption and photoluminescence measurements of thin films. WD, MDB, DC and CS

analyzed the data. MDB and CS drafted the manuscript. All the authors contributed to

interpretation of the results and revision of the manuscript. CS supervised the work. All authors

take full responsibility for the content of the paper.

Additional information

Supplementary information provided: powder X-ray diffraction measurements of the perovskite

crystals to confirm the expected perovskite structures; absorption and photoluminescence

measurements of perovskite thin films as a reference for the optical properties of the crystals;

calculation of X-ray absorption lengths of the three perovskite crystals, which turn out to be

comparable to those of commercial LaBr3 and LuI3 scintillators; time-resolved

photoluminescence of single crystals as some decay components are much faster than those in

commercial scintillators; pulse height spectra that provide additional details on scintillation

properties.

The authors declare that they have no competing financial interests. Reprints and permission

information is available online at http://npg.nature.com/reprintsandpermissions/. Correspondence

and requests for materials should be addressed to MDB (mbirowosuto@ntu.edu.sg) and CS

(csoci@ntu.edu.sg).

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26

Supplementary Information

27

X-ray Diffraction

To characterize the crystal structure of the perovskite crystals, X-ray powder diffraction (XRPD)

was performed on perovskite powders obtained from ground crystals on a BRUKER D8

ADVANCE with Bragg-Brentano geometry using Cu Kα radiation (λ = 1.54,056 Å), step increment of 0.02○ and 1 s of acquisition time. The results are shown in Figs. S1, S2 and S3.

Figure S1. XRD pattern of MAPbBr3 powders. The diffractogram is consistent

with the perovskite structure having cubic crystal system, space group Pm3m and

lattice parameters a = 5.917(1) Å.

Figure S2. XRD pattern of MAPbI3 powders. The diffractogram is consistent

with the perovskite structure having tetragonal crystal system, space group I4 ⁄

mcm and lattice constants a = 8.867(5) Å and b = 12.649(3) Å.

28

Figure S3. XRD pattern of (EDBE)PbCl4 powders. The 00l reflections indicate

the formation of the layered structure of the two-dimensional perovskite, in

agreement with the previously reported structure (by Dohner et al, [1] having

monoclinic crystal system, space group C2 and refined lattice parameters a =

7.762(8) Å, b = 7.629(2) Å, c = 13.375(7) Å, β = 102.7(2) Å.

Absorption and Photoluminescence

Absorption and photoluminescence measurements were performed in order to obtain the energy

band gap and confirm the large Stoke shift in two-dimensional perovskite scintillators (Fig. S4).

Absorption spectra were recorded by an UV-VIS-NIR spectrophotometer (UV3600, Shimadzu)

using a scanning resolution of 0.5 nm. Steady-state photoluminescence spectra were recorded by

a Fluorolog-3, (HORIBA Jobin Yvon) spectrofluorometer with wavelength resolution 0.5 nm.

Figure S4. Room temperature absorption (blue) and photoluminescence spectrum (red)

of a) MAPbI3, b) MAPbBr3, c) (EDBE)PbCl4 (thin films). The photoluminescence peak

blue-shifts from 760 nm to 527 nm from MAPbI3 to MAPbBr3, following the

corresponding blue-shift of the absorption edges. The absorption spectrum of

(EDBE)PbCl4 shows a pronounced excitonic peak at 326 nm and broadband, highly

Stoke-shifted photoluminescence peaked at 525 nm.

29

Absorption Length

Three types of the interaction mechanisms for electromagnetic radiation in matter play an

important role in the absorption of X- and -rays. These are photoelectric absorption, Compton

scattering, and pair production. All these processes lead to the partial or complete absorption of

the radiation quantum. The absorption length was obtained from formula [2]:

𝑙𝑎𝑏𝑠 ≈ 2𝑍𝑒𝑓𝑓

𝑁𝐴

1

𝜌𝜎

where Zeff, NA, , and are the effective atomic number, Avogadro number, the mass density, and the absorption cross section for each atomic element. As the cross section was separately

characterized for photoelectric, Compton scattering, and pair production, the total absorption

length was determined by the inverse sum of the absorption lengths for the three interaction

mechanisms mentioned above (Fig. S5).

Figure S5. Calculated absorption length of perovskite crystals as a function of

photon energy, covering the X-ray spectral region. Curves for Ce3+ doped LuI3 [2]

and LaBr3 [3] scintillators are also added for comparisons.

Time Resolved Photoluminescence

The microphotoluminescence setup is based on free space excitation technique with the

excitation path and the emission collection from the side, using a VIS-NIR microscope objective

(40x, NA=0.65). The MAPbI3, MAPbBr3, and (EDBE)PbCl4 single crystals were excited with 5-

MHz-repetition-rate, picosecond-pulse light sources at 640 and 370 nm of Edinburgh laser

diodes and at 330 nm of a Picoquant light-emitting diode, respectively. In all cases, the beam

spot size was about 2 mm. A silicon-based charge-coupled-device camera was used for imaging.

Time-resolved decay curves were obtained using grating Edinburgh Instruments or tunable

bandpass filters at 766, 540, and 520 nm for MAPbI3, MAPbBr3 and (EDBE)PbCl4 crystals,

respectively. The signal from the Hamamatsu photomultiplier or Micro Photon Devices single-

photon avalance photodiode was acquired by a time-correlated single photon counting card.

30

Figure S6. Time resolved photoluminescence curves of MAPbI3, MAPbBr3 and

(EDBE)PbCl4 single crystals. Excitation and emission wavelengths are reported in the

text. All intensities were normalized, and the one of (EDBE)PbCl4 was further divided by

a factor of five for clarity. The white lines in the curves are exponential fittings of the

data.

The decay curves of MAPbI3, MAPbBr3 and (EDBE)PbCl4 were fitted with double, triple, and

single exponential fits, respectively. The resulting decay components of MAPbI3 are 4.3 and 52.2

ns, with contributions of 18 and 82 %, respectively. Those of MAPbBr3 are 0.8, 5.2, and 45.4 ns

with contributions of 10, 18 and 72 %, respectively. While the longer decay times are consistent

with the values previously reported for these 3D perovskites [4], our instrumental resolution

(0.05 ns) allowed to resolve the additional presence of the fast components with decay times < 1

ns. Due to the limited time window, ultralong-lived components (> 300 ns) were not detected.

Finally, (EDBE)PbCl4 decay has only one component of 7.9 ns, consistent with the

photoluminescence lifetime reported for similar 2D perovskites [1]. Note that all the fast

components are below 10 ns, much faster than those of commercial scintillators based on Ce3+

doped LuI3 [2] and LaBr3 [3].

Pulse Height Spectra

Pulse height spectra were measured at room temperature under 662 keV gamma excitation from

a 137Cs source (no. 30/2010, 210 kBq). The pulsed output signal from a Hamamatsu R2059

photomultiplier was processed by a Canberra 2005 integrating preamplifier, a Canberra 2022

spectroscopy amplifier, and a multichannel analyzer. To improve the light collection efficiency

the samples were coupled to the quartz window of the photomultiplier with Viscasil grease and

covered with several layers of Teflon tape. Light yield is obtained from the position of the 662

keV photopeak in pulse height spectra both recorded with the photomultiplier and the APD.

Using the photomultiplier, the photoeletron yield, expressed in photoelectrons per MeV of

absorbed -ray energy (phe/MeV), is determined by comparing of the peak position of the 662

keV photopeak to the position of the mean value of the single electron response [2].

31

Figure S7. Pulse height spectra of perovskite crystals under 662 keV of 137Cs

source with a pulse shaping time of 2 s. The room temperature light yields

derived for (EDBE)PbCl4 and MAPbBr3 are 9,000 and < 1,000 photons/MeV,

respectively.

References

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[2] Birowosuto, M. D., Dorenbos, P., van Eijk, C. W. E., Krämer, K. W., and Güdel, H. U.,

High-light-output scintillator for photodiode readout: LuI3: Ce3+, J. Appl. Phys. 99,

123520-1-123520-4 (2006).

[3] van Loef, E. V. D., Dorenbos, P., van Eijk, C. W. E., Krämer, K. and Güdel, H. U.,

High-energy-resolution scintillator: Ce3+ activated LaBr3, Appl. Phys. Lett. 79, 1573-1-

1573-3 (2001).

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O. F., Sargent, E. H., Bakr, O. M., Low trap-state density and long carrier diffusion in

organolead trihalide perovskite single crystals, Science 347, 519 (2015).